1. Technical Field of the Invention
The present invention relates to an optical wavefront correction system and a method for correcting relatively severe wavefront distortions due to, for example, turbulence and more particularly to dual sensor wavefront correction system which includes a Hartmann wavefront sensor, a unit shear lateral shearing interferometer (LSI), a processing subsystem and a deformable mirror in which the processing subsystem processes the output signals from the dual sensors in a synergistic manner to form a composite correction signal that is applied to the deformable mirror resulting in a wavefront correction system with improved accuracy and dynamic range relative to known wavefront correction systems.
2. Description of the Prior Art
Optical signals are known to be distorted when passed through a time varying in-homogeneous medium, such as a turbulent atmosphere, ocean or biological tissue. Various adaptive optics systems are known which compensate the distortion in a wavefront during certain known conditions. Such adaptive optics systems normally include one or more wavefront sensors for estimating the distortion of a wavefront of an optical signal. These distortion estimates are used to generate correction signals which, in turn, are fed to the actuators of a deformable mirror in order to correct for the wavefront distortion.
Various wavefront sensors are known in the art. Such wavefront sensors are known to have limitations during certain conditions. Both unit shear lateral shearing interferometer (LSI) wavefront sensor and Hartmann wavefront sensors are extremely well known in the art. Such wavefront sensors are disclosed, for example, in "Principles of Adaptive Optics", 2.sup.nd Edition, Robert K. Tyson, Academic Press, 1991, hereby incorporated by reference.
Such sensors are also disclosed in U.S. Pat. No. 4,518,854, also incorporated by reference. In general, Hartmann sensors utilize a mask with a matrix of holes or an array of lenslets, for example, for dividing the wavefront into a matrix of subapertures. Each of the beams in the subapertures is focused onto one or more detectors forming an array of spots on the detectors. The position of the spots provides a direct indication of the wavefront tilt at each subaperture.
In unit shear LSI wavefront sensors, a copy of the wavefront is normally made and shifted in the x-direction by a distance equal to the spacing between actuators on the deformable mirror. The original and shifted beams are interfered in order to find the phase difference there between. The interference pattern is applied to an array of detectors. The intensity of the light provides a measure of the wavefront x-tilt. This is also done with a shear in the y-direction to get the y-tilt.
The output signals from the detectors from both Hartmann and unit shear LSI wavefront sensors are processed to provide tilt signal signals which, in turn, are applied to a plurality of actuators of a deformable mirror to correct for any distortions sensed in the wavefront. In particular, such output signals from such sensors are known to be processed by a so-called real reconstructor which provides relatively accurate results during conditions in which the wavefront phase change between adjacent subaperatures is .pi. or less. Examples of real reconstructors are disclosed in: "Optical Wave-Front Correction Using Slope Measurements", by Wallner; Journal of the Optical Society of America, Vol. 73, No. 12, December 1983, pages 1771-1776; "Optimal Wave-front Estimation" by Hudgin, Journal of the Optical Society of America, Vol. 67, No. 3, March 1977, pages 378-382, "Wavefront Reconstruction for Compensated Imaging", by Hudgen, Journal of the Optical Society of America, Vol. 67, No. 3, March 1977, pages 375-378; "Comparison of Wavefront Sensor Configurations Using Optimal Reconstruction and Correction", by Wallner; Proc. SPIE, Vol. 351, 1982, pages 42-53; "Phase Estimates from Slope Type Wavefront Sensors", by Noll, Journal of the Optical Society of America, Vol. 68, No. 1, January 1978, pages 139-140. "Least Square Fitting a Wavefront Distortion Estimate to An array of Phase Difference Measurements", by Fried, Journal of the Optical Society of America, Vol. 67, No. 3, March 1977, pages 370-375, all hereby incorporated by reference. However, such real reconstructors are not suitable during conditions in which the wavefront is severely scintillated due to turbulence; conditions in which the distortions are no longer continuous (i.e. conditions in which the wavefront has branch points). During such situations, such wavefront sensors processed by real reconstructors, are blind to branch point discontinuities in the wavefront. As such, wavefront sensors are known to provide rather poor results during such conditions.
There are other problems associated with such wavefront sensors. For example, one problem relates to the sensitivity of such wavefront sensor. In particular, as pointed out in U.S. Pat. No. 4,518,854, a major problem with Hartmann wavefront sensors is the alignment of the optical components and the detector array. Such misalignment can affect the sensitivity and accuracy of such a system. In order to resolve this problem, the '854 patent discloses a combined wavefront sensor which utilizes a Hartmann and an LSI wavefront sensor. The output signals from the two sensors are processed by a real reconstructor, as the generally discussed above, to provide increased sensitivity and accuracy of the wavefront sensor system. Unfortunately, such a system is unable to provide acceptable wavefront correction during conditions when a wavefront is severely distorted by turbulence as discussed above. Thus, there is a need for a wavefront correction system that is able to correct wavefronts that are severely distorted by turbulence.